31P is ubiquitous in biological systems, and the 31P NMR technique enables signals from phosphates in tissues, cells, granules or macromolecules to be monitored directly. Since the 31P NMR spectrum of ATP is sensitive to local pH, the internal pH of chromaffin granules was determined. To do this a calibration curve of 31P chemical shift pH was obtained for the internal ATP for the first time. The results gave information on the function of the granule membrane and the process of exocytosis. Similar experiments are being carried out on intact cells. The structure of the nucleosome particle of chromatin is also being investigated by this technique.